A balanced output analog differential (or "difference") amplifier circuit is a circuit to which a pair of analog input signals is applied and from which emerges an amplified pair of balanced analog output signals. That is, in response to a pair of input analog voltage signals v1 and v2, such an amplifier produces a balanced pair of amplified output analog signals V1 and V2, the output signals V1 and V2 being amplified voltages that are substantially equal in magnitude and opposite in polarity: (V1-V2)=G(v1-v2), where G is the gain of the amplifier, and, at the same time, V1+V2=0, or V1=-V2, regardless of the values of the voltages of the input signals v1 and v2. Notice that this situation of balanced outputs is a case of rejectio of common mode component by an amplifier, i.e., where in response to any two inputs v1 and v2 that are equal (v1=v2) the outputs V1 and V2 are both equal to zero: V1=V2=0.
Notice also that a balanced output amplifier (V1=-V2) ensures that in response to any increments Dv1 and Dv2 in the inputs v1 and v2, respectively, the corresponding increments DV1 and DV2 in the outputs will substantially satisfy the relation DV1+DV2=0 or DV1=-DV2. Thus, any nonlinearity in the amplifier's response which would otherwise make the absolute magnitude of DV1 differ from that of DV2 is compensated, so that the absolute magnitudes of DV1 and DV2 are always substantially equal in the case of the balanced output amplifier. In other words, in a balanced output amplifier, the common mode output component is suppressed whether it is caused by common mode in the input or by internal sources of common mode, such as nonlinearity of the amplifier's response.
A balanced output differential amplifier, because of its two balanced outputs, can be used to advantage in conjunction with symmetric circuit topologies. For example, in the field of analog integrated circuits, symmetric networks have been used to improve both the dynamic range and the power supply noise rejection of integrated circuit filters, both of the continuous time and of the switched capacitor variety, and the balanced output differential amplifier can thus be used to advantage in such integrated circuit filters for the purpose of amplifying signals therein.
In U.S. Pat. No. 3,786,362, a balanced output analog differential amplifier circuit is described in which a common mode detector of the common mode component in the output supplies a feedback control signal to a pair of variable loads, the loads being connected in an input stage of the amplier circuit. The feedback signal varies the respective impedances of the loads, and hence varies their respective voltage drops, so that the common mode component tends to be suppressed. This amplifier circuit, however, suffers from the problem that the feedback becomes inoperative in suppressing the common mode in the output when the input signals are such that they turn off both of the transistors (in the amplifier's input stage) to which the input signals are applied.